Ok, the good news is that a pair of dippers are using the nesting tube where the camera is installed. I checked the camera and it works perfectly. The problem is the dippers are nesting at the wrong end of the nesting tube. After spending a bit of time watching them I figured out why they have switched ends, its to do with the rocks exposed in the river. The dippers prefer to have a rock in the river where they can launch themselves directly up to the nest after checking there are no predators about. Due to the large amounts of rain we have had recently the river is running high and the rock the dippers normally use is completely submerged. There is a rock exposed on the other side of the nesting tube which they can use to launch themselves up the nest. As a result they have swapped ends in the nesting tube. I will have to wait until next year and ensure that there is a rock high enough in the water for the dippers to use. Nobody said this would be easy!